There are pre-overclocked versions of this card available. You could check their rated power consumption against clock speed to get a good idea of how far you could go.
FWIW I decided against trying this myself. I figure the PCIe speed in my Mac Pro (2010) is the limiting factor now. Really enjoying the upgrade (from 5770), but I'm not really wowed. Hopefully driver updates will improve the performance over time.

Do you really need the overclock?
As it would require more power and it would produce more heat.

Afterall you will get only a few more FPS ingame, so if you wanna play a game that is laggy because of the low FPS then those 5 extra frames per second aren't gonna help you much.
At the moment GTX680 is a very powerful card.

To answer your question, 195w is probably the maximum it would draw (at current clockspeed) but also keep some headroom available for peakloads.
The PSU can deliver 225w but if you are in a region with low mains line then the PSU will most likely not deliver 18,7ampere (225watt) into the 12v line steadily.